Silicon carbide in the form of whiskers with a high aspect ratio of length to diameter has been proposed for use as a reinforcement in aluminum metal to increase its modulus of elasticity and greatly enhance its utility for the most critical aerospace applications. It has been reported that an addition of 25 volume % of SiC powder, of which about 20% was in the form of whiskers, increased the Young's modulus of the standard aircraft alloy 2024-T4 by approximately 100%, and the ultimate tensile strength by approximately 50%.
Silicon carbide occurs in two crystal forms, a .beta. form which is cubic and an .alpha. form which crystalizes in both hexagonal and rhombohedral classes in a number of polytypes. Although there are broad temperature ranges in which either may form, in general, temperatures higher than 1900.degree. C. favor the formation of the .alpha. form and temperatures between 1200.degree. to 1900.degree. C. favor the formation of the .beta. form. The .alpha. type is preferred as an abrasive due to its hardness, while the .beta. form has been of little commercial importance.
SiC whiskers have been made by the methods disclosed by Cutler in U.S. Pat. No. 3,754,076, (1973); by Adamsky and Merz, Z. Krist 111, 350-361 (1959); and by Hamilton, J. Appl. Phys 31, 112-116 (1960); see Kirk-Othmer Enc. Chem. Tech. Vol. 4, 2nd Ed. Wiley & Sons, N.Y., 1964, (117-118). U.S. Pat. No. 3,269,802 by Wainer et al., Aug. 30, 1966, Class 23-349 discloses the preparation of carbide products by carbonizing filaments or felts then converting the carbonized product in an atmosphere of a halide or carbonyl of the carbide forming element. U.S. Pat. No. 3,403,008 by Hamling, class 23-344, Sept. 24, 1968, discloses the production of metal carbide fibers, textiles, and shaped articles by impregnating a pre-formed organic polymeric material with a solution of a metal compound, heating the impregnated material to leave a carbonaceous relic containing the metal in a finely dispersed form and further heating the relic to 1000.degree. to 2000.degree. C. in a non-oxidizing atmosphere to form the metal carbide. U.S. Pat. No. 3,640,693 by Galasso, class 55/2, Feb. 8, 1972, discloses a process for producing high modulus fibers by forming a silicon metal fiber in a glass tube to form a composite, removing the glass sheath and contacting the exposed silicon core with a suitable reactant such as methane to form the SiC fiber. U.S. Pat. No. 4,127,659 by DeBolt et al., Class 427/249, Nov. 28, 1978, discloses a refractory substrate of graphite or carbon overcoated with SiC by deposition from gaseous sources of silicon in combination with hydrogen. British Pat. No. 998,089 by Evans et al., Int. Cl:-C01b, July 14, 1965, discloses the production of refractory carbide fiber by heating a cloth woven from cellulosic or other organic material such as viscose rayon in an inert atmosphere. The carbon cloth is then completely imbedded in fine powdered silicon and heated in an inert atmosphere or argon to 1410.degree. C. for between 4 and 24 hours, forming a SiC cloth in the original shape. Japanese Pat. No. 52/113,300 by Tokai Carbon K.K., Cl. C01b-31/36, Oct. 3, 1978, discloses heating rice chaff in a vessel by high-frequency radiation in a tube-type furnace under CO atmosphere at 1700.degree.-2000.degree. C. to form .alpha.-SiC whiskers.
U.S. Pat. No. 3,412,062, Johnson, Nov. 19, 1968, discloses carbon fibers from PAN polymers. U.S. Pat. No. 3,503,708, Spry, Mar. 31, 1970, discloses carbon fibers produced from rayon. U.S. Pat. No. 4,014,725, Schulz, Mar. 29, 1977, discloses mesophase pitch based carbon fibers. U.S. Pat. No. 3,285,696, Tsunoda, Nov. 16, 1966, discloses PAN copolymer based carbon fibers.
Considerable scientific study of whiskers has been done, and their properties have been found to be of great utility in reinforcing lower modulus solids, but commercial development has been limited due to the extremely high cost of production. Typically they are produced in gas-phase reactions with low yields in expensive processes, which has limited availability to that of laboratory samples.
One of the problems unsolved until the present invention was that of obtaining a reasonable yield of whiskers. When attempting to manufacture whiskers, about 20-25% of the SiC obtained is the maximum obtained in the whisker form, the rest being irregularly shaped particles of little or no value as reinforcement.
It is difficult to separate the whiskers from the powder due to their small size. Mechanical screening is slow and imperfect, while air classification has so far been unsuccessful. Consequently, it is imperative to have the whiskers in high yield if the material is ever to become a large scale commercial product.